Low pass filters are widely used to extract spatial components other than noise components from image signals in order to suppress the noise components in the image signals, but low pass filters have the drawback of also suppressing edge components of subject images in the image signals. Japanese Patent Application Publication No. 2007-188346 (Patent Reference 1), for example, discloses an image processor that can suppress noise components in an image signal while preserving edge components in the subject image.
FIG. 1 is a functional block diagram showing the configuration of an image processor 100 disclosed in Patent Reference 1. As shown in FIG. 1, the image processor 100 has low pass filters 101, 102, 103 that execute filtering on an input image signal, difference calculators 104, 105, 106, base noise clip circuits 107, 108, 109, and an adder 110. Difference calculator 104 outputs the difference between the input image signal and the output of low pass filter 101, difference calculator 105 outputs the difference between the outputs of low pass filters 101 and 102, and difference calculator 106 outputs the difference between the outputs of low pass filters 102 and 103. The low pass filters 102 and 103. The low pass filters 101, 102, 103 execute filtering with reference to the pixels in mutually differing ranges centered on a pixel of interest, so the difference calculators 104, 105, 106 may output signals with mutually differing spatial frequency components. The base noise clip circuits 107, 108, 109 receive the output signals of the difference calculators 107, 108, 109 and output fixed values when the received values are within prescribed ranges defined by threshold values (clip levels). Adder 110 adds the outputs of these base noise clip circuits 107, 108, and 109 to generate a pixel corresponding to the pixel of interest. Noise components in the image signal can be suppressed while preserving edge components of the subject image by appropriately setting the clip levels in the base noise clip circuits 107, 108, and 109.